1. Field of the Invention
The present invention relates to a method of producing a semiconductor device, and particularly, to a method of producing a semiconductor device comprising device isolation structure.
2. Related Art
There has been known a shallow trench isolation (STI) as device isolation structure in a semiconductor device. The STI structure is formed such that an insulating film is deposited on a semiconductor substrate in which trenches are formed to bury the trenches with the insulating film. A chemical vapor deposition (CVD) method is generally used to deposit the insulating film.
Hitherto, an insulating film has been deposited by a high-density plasma (HDP) CVD method, for example. However, further miniaturization of a device has increased the aspect ratio of a trench, which has made it difficult to bury trenches by the HDP-CVD method. For this reason, other vapor deposition methods having more excellent coverage has drawn attention instead of the HDP-CVD method in recent years. Such vapor deposition methods include a sub-atmospheric (SA) CVD method using gases containing O3 (ozone)-TEOS (tetraethoxysilane) and an atomic layer deposition (ALD) method. These methods are excellent in coverage and characterized in that a conformal thin film can be formed.
A method of forming a semiconductor device according to a related art will be described below with reference to FIGS. 1A to 1C and FIGS. 2A to 2E.
First, an oxide film (not shown) is formed on semiconductor substrate (semiconductor wafer) 1 by a thermal oxidation process. Then, silicon nitride film (SiN film) 2 is formed by a low-pressure (LP) CVD method. At this point, SiN film 2 is deposited on both sides of semiconductor substrate 1 as illustrated in FIG. 1A. That is to say, on front surface FS of semiconductor substrate 1 is formed SiN film 2a, and on back surface BF of semiconductor substrate 1 is formed SiN film 2b. SiN film 2 exerts a strong tensile stress on semiconductor substrate 1. However, the stress is in balance on the front and the back surface in a phase illustrated in FIG. 1, so that “warpage” does not appear on semiconductor substrate 1.
A mask is formed from SiN film 2a on front surface FS of semiconductor substrate 1 through photolithography and etching. The mask is used to form an STI structure and has a pattern corresponding to the STI structure. SiN film 2a, as illustrated in FIG. 1B, is segmented by pattering to release the stress. As a result, stress is imbalanced between front surface FS and back surface BS to convexly warp front surface FS of semiconductor substrate 1. In other words, a tensile stress of SiN film 2b on back surface BS convexly warps semiconductor substrate 1.
Exposed portions on semiconductor substrate 1 are subjected to dry etching using the mask to form trenches 3 in front surface FS of semiconductor substrate 1 as illustrated in FIG. 1B. Subsequently, oxidation treatment is performed to form 10 to 15 nm thick oxide film 4 on the inner walls of trenches 3. After that, 6 to 8 nm thick SiN film is formed as liner film (not shown). The liner film plays a role to suppress the oxidation of semiconductor substrate 1 at the following steam anneal process and oxidation process.
As illustrated in FIG. 1C, oxide film 5 is deposited to cover front surface FS of semiconductor substrate 1 by the SA-CVD method using gases containing O3-TEOS. As a result, trenches 3 are buried with oxide film 5. Oxide film 5 deposited by this method is referred to as “O3-TEOS-USG (undoped silicate glass).” The above SA-CVD method has an excellent coverage, and O3-TEOS-USG to be formed is a conformal film.
Care must be devoted to portions poor in film quality referred to as “seam” when trenches 3 are buried with the conformal film. The term “seam” refers to a line where the edges of two films growing from both side walls in trench 3 touch each other. As illustrated in FIG. 1C, oxide films 5 deposited in each trench 3 are joined with each other to form seam SE in each trench 3. Large aspect ratio of trench 3 makes seams SE particularly conspicuous. The etching rate of wet etching to seams SE is higher than that to other portions of the film, which is unfavorable for processing.
Then, annealing is performed in an atmosphere of water vapor at temperatures of 700° C. to 950° C. (steam anneal process) to improve the film quality of the entire oxide film 5 including seams SE. Furthermore, annealing is performed in an atmosphere of inert gas (N2) at a temperature of 1100° C. This intends oxide film 5 to be densified. However, seams SE cannot be completely removed by such an annealing process in the related art. Particularly, in recent years, the aspect ratio of trench 3 has tended to increase in accordance with miniaturization of a device, so that it is very difficult to eliminate seams SE.
FIG. 2A illustrates an enlarged periphery of trench 3. As illustrated in FIG. 2A, trenches 3 are formed on front surface FS of semiconductor substrate 1. Oxide film 4 is formed on the inner wall of trench 3. Oxide film 5 is formed to bury trenches 3. Seams SE are formed at positions corresponding to trenches 3 inside oxide film 5.
As illustrated in FIG. 2B, chemical mechanical polishing (CMP) is performed to such an extent that SiN film 2a is exposed. As a result, STI structure 6 is formed in which trench 3 is buried with oxide film 5. Seams SE exist in STI structure 6.
As illustrated in FIG. 2C, oxide film 5 between adjacent masks (SiN film 2a) is removed by wet etching using fluorinated acid. The etching rate of wet etching to seams SE is higher than that to other portions, so that oxide film 5 around seam SE is etched faster. As a result, slit dents DE are formed at positions corresponding to seams SE on the surface of STI structure 6.
As illustrated in FIG. 2D, SiN film 2a used as a mask is removed by etching. Wet etching is performed by, for example, phosphoric acid heated to temperatures of 140° C. to 160° C. In this process, SiN film 2b formed on back surface BS of semiconductor substrate 1 is also removed at the same time. Incidentally, dents DE still remain on the surface of STI structure 6.
As illustrated in FIG. 2E, gate oxide film (not shown), gate polysilicon film 7, tungsten film 8 and SiN film 9 are sequentially deposited in this order. After that, a gate electrode of an MOS transistor is formed by photolithography technique and dry etching technique.
However, in this case, a conductor (i.e., gate polysilicon film 7) used as a material of the gate electrode is deposited also in dents DE. Even after a gate electrode is formed by dry etching, the conductor in dents DE may not be completely removed. The conductor remaining inside dents DE causes a short circuit between adjacent gate electrodes to deteriorate yield. Better technique is demanded to solve problems resulting from seams SE in oxide film 5.
There have been known the following techniques for solving problems resulting from a stress exerted on a semiconductor substrate.
Japanese Patent Laid-Open No. 2004-158711 describes that STI structure is formed with an SOI wafer concavely warped and thereafter the warp of the SOI wafer is corrected to generate a crack in the STI structure. The concave warp results from a compressive stress film (or a polycrystalline silicon film) formed on the back surface of the SOI wafer. According to a technique for solving the problem described in this document, the compressive stress film on the back surface of the SOI wafer is removed and then the STI structure is formed.
Japanese Patent Laid-Open No. 2005-340734 describes a technique for decreasing a stress resulting from a silicon nitride film for forming the side wall of a gate electrode. When a silicon nitride film is deposited, it is formed not only on the front surface of a semiconductor substrate, but also on the back surface thereof. The above document indicates that the stress resulting from the silicon nitride film on the back surface thereof deteriorates characteristics of an MOS transistor. According to the technique for solving the problem described in this document, the silicon nitride film is deposited and then an antifouling oxide silicon film is selectively formed only on the front surface of the semiconductor substrate. The silicon oxide film is used as a mask to selectively etch the silicon nitride film on the back surface thereof. After that, the silicon oxide film on the front surface is selectively etched. Then, the silicon nitride film on the front surface is subjected to anisotropic etching to form the side wall of a gate electrode.